1. Field of the Invention
This invention relates to a catalyst for hydrorefining hydrocarbon feedstocks and to a hydrorefining process which makes use of the said catalyst.
2. Description of the Related Art
Petroleum and petroleum fractions are highly complex mixtures in which, besides the hydrocarbons, there are present compounds containing heteroatoms such as sulphur, nitrogen and oxygen. These are generally impurities which are detrimental to the good quality of the petroleum products. In particular, they are responsible for problems related to pollution, corrosion, smell and stability. It is therefore important to remove them during the refining. This generally involves a treatment with hydrogen in the presence of a catalyst which promotes the rupture of the carbon-heteroatom bonds. This treatment is called hydrorefining.
Considerable research effort has been deployed to find efficient hydrorefining catalysts. The catalysts which are most commonly employed are based on nickel, molybdenum, tungsten or cobalt.
The remainder is prepared by impregnating alumina, generally gamma alumina, with an aqueous solution of metal salts. After impregnation the support is dried and then calcined.
The conversion of the metal oxides formed, into sulphides, takes place in the presence of sulphur compounds such as hydrogen sulphide, carbon disulphide, elemental sulphur, mercaptans or thioethers. This sulphurisation generally takes place in the presence of hydrogen.
Until now, niobium sulphide has stimulated little interest as a hydrorefining catalyst. Pecorado and Chianelli (J. Catal. 67 430 (1981)) have shown that bulk niobium sulphide has a weak activity for the hydrodesulphurisation of dibenzothiophene. Similarly, carbon-supported niobium sulphide is very weakly active in hydrodesulphurisation (Vissers, Bull. Soc. Chim. Belg. 93 813 (1984)) and in hydrodenitrogenation.
In all the studies the material involved is niobium disulphide, which is the most stable sulphur derivative of niobium and the one easiest to synthesize.
We have now found that another niobium sulphide, the trisulphide, which is difficult to synthesize and unstable in an atmosphere which is not rich in sulphur, exhibits remarkable catalytic properties.